Environmental Engineering Reference
In-Depth Information
FAO-listed food and feed crops harvested from i elds as annual crops or gathered
from permanent plantations thus added up to nearly 5.7 Gt of fresh phytomass.
Fiber crops (cotton, l ax, hemp, jute, ramie, and sisal) are a minor addition: their
fresh mass (about 25 Mt in the year 2000) is much lower than a reporting error for
tubers or cereals. Agricultural land is also used to grow forages, including silage
(mainly corn and sorghum) and leguminous cover crops (mainly alfalfa and clover
in North America and Europe and vetches in Asia), often in rotation with other feed
or food crops.
The FAO does not publish any regular statistics regarding these harvests, and
annual outputs have to be pieced from data published by major agricultural produc-
ers and then extrapolated in order to account for production in countries that
publish no relevant information: consequently, all of these annual rates have large
(more than 25%) margins of uncertainty. American corn silage production has been
on the order of 100 Mt/year, France (Europe's greatest producer) every year grows
about 20 Mt of corn and grass silage, and the worldwide output has been close to
250 Mt/year. National totals for hay produced on arable land in the year 2000 were
about 140 Mt of fresh weight in the United States and close to 20 Mt in France,
and the global output has been on the order of 1 Gt/year.
And then there is a specii c accounting problem posed by green manures. These
leguminous cover crops (alfalfa, clover, vetch, beans) are used in rotations (often
also as forages) and then worked into soil. This recycling takes place usually 60-120
days after their planting, and in temperate latitudes it can add enough nitrogen to
produce a good summer cereal crop. This practice was common in all traditional
intensive agricultures: in China it peaked as recently as 1975 with plantings of nearly
10 Mha of green manures before the availability of affordable synthetic nitrogenous
fertilizers brought its rapid decline (Smil 2004). How should the phytomass produc-
tion of green manures be counted? Obviously, their phytomass is not directly appro-
priated, and the nitrogen they i x is available not only to a subsequent nonleguminous
crop but also to soil bacteria and soil invertebrates.
The only adjustment that has to be done for all production data to make them
comparable across many species is to convert them from fresh-weight basis to abso-
lutely dry weight. Arithmetically this is just a matter of simple multiplication—but
also another occasion for introducing inevitable errors. But these errors are bound
to be considerably smaller than those arising from the often questionable reporting
of many national crop harvests or their estimates made in the FAO's Rome head-
quarters, or from the necessity to make nothing better than rough estimates of
animal fodder produced from agricultural land. For food and feed crops the conver-
